Thread-Safe ArrayList merge: will this work?

Question

  private static final ExecutorService ES = Executors.newWorkStealingPool();

  public Future<List<String>> isThisSafe() {
    List<String> a = new ArrayList<>();
    a.add("a");
    List<String> b = new ArrayList<>();
    b.add("b");

    return ES.submit(() -> {
      a.addAll(b);
      return a;
    });
  }

I'll say that thread X is the one that calls isThisSafe(). Thread Y runs the Callable submitted to the ExecutorService.

Right now, I think this works.

✓ The two ArrayLists are never modified by X after being published to Y

✓ ArrayList 'a' is modified by thread Y, but X does not hold a reference to 'a'.. the returned reference to 'a' would be considered 'new' and so X will pull this from MM.

But is there a problem with my second point? Maybe I misunderstand the memory model.

Answer 1

The thread X safely publishes the object a to thread Y and does not do any modification until Y succeeds. So the object a is safely published from thread X to thread Y.

Moreover, All actions in a thread happen-before any other thread successfully returns from a Future.get() on that thread. Upon returning from Future.get() thread X can see all the changes made by thread Y to the object a.

So according to our thread safety analysis, we can conclude that this code is thread safe.